[FieldTrip] Beamformer: different length of baseline and post baseline interval

[Akiko Ikkai]

Hi,

I know this is not a recent post, but please allow me to ask a follow-up
question; can you use cfg.pad for ft_freqanalysis for the baseline in this
case (assuming post-baseline period of interest is uniform length, say
1000ms)?

Such as:
% extract baseline (500ms)
cfg             = [];
cfg.toilim      = [-.7 -.2];
data_BL         = ft_redefinetrial(cfg,ft_data);

% pad to make it 1000ms and run ft_freqanalysis
cfg             = [];
*cfg.pad         = 1;*
cfg.method      = 'mtmfft';
cfg.output      = 'powandcsd';
cfg.foi         = foi;
cfg.taper       = 'dpss';
cfg.tapsmofrq   = smooth;
freq_BL         = ft_freqanalysis(cfg,data_BL);

Thanks! Akiko

On Mon, Jul 2, 2012 at 4:07 PM, Johanna Zumer
<johanna.zumer at donders.ru.nl>wrote:

> Dear Anna,
>
> Ideally for the common filter, you want the same amount of data T(s) per
> condition, where T = N x tw (and N is number of trials and tw is timewindow
> length).  In your case, if the baselines for each conditions can be
> combined into one general baseline, and if you happen to have 100 trials
> per condition, then T_baseline = 3 x 100 x 0.5s = 150s.  If you then use
> 1.5s length post-baseline, then T_each_condition = 100 x 1.5s = 150s, so
> you now have equal T for each condition for the common filter.
>
> However, in order to have an equal effect of tapers and edge-effects on
> the different conditions, you should use equal time window lengths in
> freqanalysis.  Thus it would be better to split your post-baseline data
> into 3 segments of 500ms each before calling ft_freqanalysis, which again
> gives T = 100 x 0.5 x 3 = 150s.
>
> Cheers,
> Johanna
>
> 2012/6/29 Anna Wilsch <wilsch at cbs.mpg.de>
>
>>
>> Dear Fieldtrippers,
>>
>> I'm trying to beamform my MEG data by building a common filter including
>> three conditions and a baseline for each condition. The baseline intervals
>> have a duration of 500 ms. I was wondering if it is ok if the post-baseline
>> data are longer than that (1000 - 2000 ms). Does it have any negative
>> impact on the cross-spectral-density matrix and/or the common filter? Would
>> that still be a valid operation to do or is it necessary that baseline and
>> post baseline data have the same length?
>> Thank you for your comments.
>>
>> Cheers,
>> Anna
>>
>>
>> Anna Wilsch, Dipl.-Psych.
>> Auditory Cognition Research Group
>> Max Planck Institute for Human Cognitive and Brain Sciences
>> Stephanstr. 1a - Leipzig, Germany
>> (p) +49 (0)341 9940 2641
>> (e) wilsch at cbs.mpg.de
>>
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>>
>
>
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-- 
Akiko Ikkai, Ph.D.
Postdoctoral Fellow
Department of Psychological and Brain Sciences
Johns Hopkins University
Ames Hall, 3400 N. Charles St.
Baltimore, MD 21218
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